1. Field of the Invention
The present invention relates to a photoelectric conversion method, a light receiving circuit and an optical communication system of an analog optical modulated signal, especially to a photoelectric conversion method, a light receiving circuit and an optical communication system in which a constant power level of demodulated signal can always be obtained.
2. Description of the Related Art
In a cellular phone system or another mobile communication system, analog optical transmission is performed in which a high-frequency signal for mobile communication is converted to light and transmitted via an optical fiber or the like.
In the analog optical transmission, for example, when a light signal transmitted from one plate needs to be simultaneously received in a plurality of places and converted to an electric signal, a passive light element called a star photocoupler is used to distribute the light signal, which is further transmitted via optical fibers or the like.
The star photocoupler is ideally an equipartition unit, but actually has a distribution deviation of about xc2x11 to 2 dB.
Moreover, the distributed light signals are transmitted through the optical fibers to reach the light receiving circuits. However, since the transmission distance via the optical fiber varies with the light receiving circuit, the propagation loss of light differs after distribution.
Furthermore, since the connection loss or the like of an optical connector or another optical connection portion is not identical, the average photoelectric power Pr incident upon each light receiving circuit differs.
In the analog optical transmission, CNR (carrier to noise ratio) of the received signal is represented by equation 1. In the equation, the relative intensity of light source noise (RIN), the optical modulation intensity (OMI), the photoelectric conversion efficiency of a light receiving element (xcex7), the input conversion noise current density of an initial-stage amplifier (Ith), and a noise bandwidth (BW) are used.                     CNR        =                                            1              2                        ⁢                                          (                                  OMI                  ·                  η                  ·                  Pr                                )                            2                                                          {                                                RIN                  ·                                                            (                                              η                        ·                        Pr                                            )                                        2                                                  +                                  2                  ·                  q                  ·                                      (                                          η                      ·                      Pr                                        )                                                  +                                  ⟨                                                            Ith                      _                                        2                                    ⟩                                            }                        ⁢            BW                                              [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
It is found in the equation 1 that if the relative intensity of light source noise (RIN), the optical modulation intensity (OMI), the photoelectric conversion efficiency of the light receiving element (xcex7), the input conversion noise current density of the initial-stage amplifier (Ith), the noise bandwidth (BW) and other parameters are constant, the received CNR largely changes with the average received light level Pr.
Therefore, for the light signal transmitted from one place as described above, if the average photoelectric power Pr incident on the light receiving circuit differs with the distribution deviation at the time of distribution via the star photocoupler, the difference of transmission loss in the course of optical transmission and the difference of the connection loss of the photo connecting portion, the received CNR after optical demodulation or the output power level of the electric signal (corresponding to the numerator of the equation 1) differs.
As derived from the equation 1, the fluctuation of xc2x11 dB of received light power corresponds to the fluctuation of xc2x12 dB (sign of the same order) at the output power of the electric signal. Therefore, when the signal level of the electric signal demodulated from the light signal needs to be set the same at each light receiving terminal, some level compensating means is necessary.
Furthermore, the necessity of level correction will be described in detail.
In the analog optical transmission system of high-frequency signals for mobile communication such as a cellular phone, high-frequency signals are restored after the optical transmission, and need to be radiated to the space in the form of radio waves via antennas or the like.
When radio waves are radiated to the space, transmission power, spurious radiation, and the like are regulated according to an ordinance. In the analog optical transmission, the average received light power largely influences the power level of the high-frequency signal after optical demodulation. Specifically, as described above, the difference of 1 dB in the average received light power is enlarged to 2 dB as the high-frequency signal power level after the optical demodulation.
Ideally, when there is no distribution deviation of a light distributor, or there is a uniform transmission loss to each substation after distribution, the average received light power might be the same, which is not realistic.
In the existing circumstances, it is often unknown in the stage of apparatus manufacture the optical fiber with what degree of loss is to be connected to which port of the light distributor. Therefore, the gain regulation attributed to these factors (for adjusting the high-frequency output (transmission power) of each substation to the normal value) needs to be handled on the site after installation.
Moreover, if such value is known beforehand, it can be individually adjusted. In this case, however, the degree of freedom in the installation of the substation is remarkably lowered after the regulation. Specifically, only a specified substation must be connected to a specified branch output under constrains.
To solve these problems, a circuit has been heretofore used in which an additional function of regulating the output level (variable attenuator) is provided behind the light receiving circuit.
The conventional light receiving circuit will be described with reference to FIG. 12. FIG. 12 is a circuit diagram of the conventional light receiving circuit.
In the conventional light receiving circuit, a light receiving diode 1 is connected to a resistance 2 in series, a direct current voltage +Vb is applied to the anode side of the light receiving diode 1, and the other end of the resistance 2 is connected to the earth (grounded). Furthermore, a capacitor 3 whose one end is grounded is connected in parallel between the anode side of the light receiving diode 1 and the direct current voltage +Vb.
Moreover, a demodulation means constituted of a capacitor 4, an initial-stage amplifier 5, a main amplifier 6 and a variable attenuator 7 is connected to the connection point of the cathode side of the light receiving diode 1 and the resistance 2.
Here, the attenuation level in the variable attenuator 7 is variable, but needs to be preset. For example, the average loss or the like in the process of optical transmission is measured when the light receiving circuit is installed, and the attenuation level is adapted to the measurement.
In operation of the conventional light receiving circuit, the light signal subjected to the analog optical modulation is transmitted via an optical fiber to form a light signal P, which is received by the light receiving diode 1 and photoelectrically converted to form a high-frequency electric signal. Its alternating component (i.e., a transmitted signal obtained by demodulation) is amplified in the initial-stage amplifier 5 and the main amplifier 6 via the capacitor 4 and transmitted to the variable attenuator 7.
Subsequently, a constant level of demodulated output is obtained by performing attenuation in the variable attenuator 7 with the preset attenuation level.
However, in the aforementioned conventional light receiving circuit, once the attenuation level in the variable attenuator 7 is set, it is fixed. Therefore, if the average received light level of the analog modulated light signal received by the light receiving diode 1 varies, the output level of the demodulated signal also varies, which causes a problem that a constant output level cannot be obtained.
An object of the present invention is to provide a photoelectric conversion method, a light receiving circuit and an optical communication system in which a constant output level of demodulated signal can always be obtained even if the average received light level varies.
In the photoelectric conversion method of the present invention, a received light is converted to an electric signal, the average received light level of the received light is detected, and the electric signal is amplified or attenuated based on the detected average received light level to obtain a demodulated signal. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Moreover, in the photoelectric conversion method of the present invention, a received light is converted to an electric signal, an alternating component is removed and a direct current component is taken out from the received light, the average received light level of the light is detected using the direct current component, a difference of the detected average received light level and the preset reference voltage is amplified to use the amplified voltage as a control voltage, and the electric signal is amplified or attenuated with the gain corresponding to the control voltage to obtain a demodulated signal. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Furthermore, in the photoelectric conversion method of the present invention, a received light is converted to an electric signal, an alternating component is removed and a direct current component is taken out from the received light, the average received light level of the light is detected using the direct current component, the detected average received light level is shifted based on the preset reference voltage to use the shifted voltage as a control voltage, and the electric signal is amplified or attenuated with the gain corresponding to the control voltage to obtain a demodulated signal. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Moreover, in the photoelectric conversion method of the present invention, a received light is converted to an electric signal, a received light voltage value obtained from the received light is converted to a digital value, the digital value is averaged to obtain an average received light level, a difference of the average received light level and the preset reference voltage value is calculated, a control voltage value is obtained corresponding to the difference, the control voltage value is converted to an analog signal, and the electric signal is amplified or attenuated with the gain corresponding to the analog signal to obtain a demodulated signal. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Additionally, in the present invention, the light receiving circuit comprises a light receiving element to which a voltage is applied, a capacitor connected to one end of the light receiving element for removing a direct current component of an electric signal generated in response to a light received by the light receiving element and outputting an alternating component, a low-pass filter connected to the other end of the light receiving element for removing the alternating component of the electric signal generated in response to the light received by the light receiving element to output the direct current component as an average received light level, a differential amplifier for amplifying a difference of the average received light level outputted from the low-pass filter and the preset reference voltage, and a variable gain amplifier for receiving a voltage of the amplified difference as a control voltage to amplify or attenuate the electric signal of the alternating component outputted from the capacitor with a gain corresponding to the control voltage. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Moreover, the light receiving circuit of the present invention comprises a light receiving element to which a voltage is applied, a capacitor connected to one end of the light receiving element for removing a direct current component of an electric signal generated in response to a light received by the light receiving element to output an alternating component, a low-pass filter connected to the other end of the light receiving element for removing the alternating component of the electric signal generated in response to the light received by the light receiving element to output the direct current component as an average received light level, a level shift circuit for shifting the average received light level outputted from the low-pass filter based on the preset reference voltage, and a variable gain amplifier for receiving a voltage of the shifted level as a control voltage to amplify or attenuate the electric signal of the alternating component outputted from the capacitor with a gain corresponding to the control voltage. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Furthermore, the light receiving circuit of the present invention comprises a light receiving element to which a voltage is applied, a capacitor connected to one end of the light receiving element for removing a direct current component of an electric signal generated in response to a light received by the light receiving element to output an alternating component, an A/D converter connected to the other end of the light receiving element for converting the electric signal generated in response to the light received by the light receiving element to a digital signal to output the digital signal, a controller for averaging the digital signal to calculate an average received light level and to obtain a control voltage value corresponding to a difference of the average received light level and the preset reference voltage value, a D/A converter for converting the control voltage value to an analog signal to output the analog signal, and a variable gain amplifier for receiving a voltage of the analog signal as a control voltage to amplify or attenuate the electric signal of the alternating component outputted from the capacitor with a gain corresponding to the control voltage. The average received light level can be reflected on the amplification or attenuation of the electric signal.
Moreover, in the light receiving circuit of the present invention, an electronic attenuator may be used instead of the variable gain amplifier.
Furthermore, in the light receiving circuit of the present invention, an IC filter may be used instead of the low-pass filter.
Additionally, in the light receiving circuit of the present invention, the differential amplifier for amplifying the electric signal generated on the light receiving element is provided between the other end of the light receiving element and the low-pass filter. Even if the average received light level is small, the differential amplifier or the level shift circuit can be operated stably.
Moreover, in the light receiving circuit of the present invention, the differential amplifier for amplifying the electric signal generated on the light receiving element is provided between the other end of the light receiving element and the IC filter. Even if the average received light level is small, the differential amplifier or the level shift circuit can be operated stably.
Furthermore, in the light receiving circuit of the present invention, an operating amplifier may be used instead of the level shift circuit.
Moreover, the optical communication system of the present invention comprises a high-frequency source for generating a high-frequency signal, an electric/optical conversion circuit for performing analog optical modulation of the high-frequency signal to output a light signal, a star photocoupler for distributing the light signal, an optical fiber for transmitting the distributed light signal, and the light receiving circuit for demodulating the transmitted light signal to obtain a demodulated signal. Even if the light signal is transmitted with a transmission loss which varies with a path, and differs in the average received light level in each light receiving circuit, a constant output level can be obtained. Because the electric signal is amplified (attenuated) with the gain corresponding to the average received light level in each light receiving circuit to obtain the demodulated signal.